>>>>> Jeffrey Dick on Wed, 5 Nov 2025 15:54:36 +0800 writes:
> Setosa sepals are bigger than petals in both length and width:
> summary(subset(iris, Species == "setosa"))
> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
> Min. :4.300 Min. :2.300 Min. :1.000 Min. :0.100 setosa
:50
> 1st Qu.:4.800 1st Qu.:3.200 1st Qu.:1.400 1st Qu.:0.200
versicolor: 0
> Median :5.000 Median :3.400 Median :1.500 Median :0.200 virginica
: 0
> Mean :5.006 Mean :3.428 Mean :1.462 Mean :0.246
> 3rd Qu.:5.200 3rd Qu.:3.675 3rd Qu.:1.575 3rd Qu.:0.300
> Max. :5.800 Max. :4.400 Max. :1.900 Max. :0.600
> However, in the first example with iris in ?matplot, setosa petals (small
red "s") appear to be longer and wider than sepals (big blue "S"). The symbols
for versicolor are also swapped. (I'm using R 4.5.2.)
> A possible fix is to change legend() to put "Sepals" before "Petals" for
both species. This ordering corresponds with the column indexing used in the
example:
> iS <- iris$Species == "setosa"
> names(iris[iS,c(1,3)]) == c("Sepal.Length", "Petal.Length")
> names(iris[iS,c(2,4)]) == c("Sepal.Width", "Petal.Width")
> The original legend used bigger (uppercase) symbols for bigger flower
parts. Whether or not this was intentional, it is an intuitive choice. Such a
congruence can be maintained by changing "sS" to "Ss" and "vV" to "Vv" in all
three pch assignments.
> Here is a modified example with the proposed changes in legend and pch.
> table(iris$Species) # is data.frame with 'Species' factor
> iS <- iris$Species == "setosa"
> iV <- iris$Species == "versicolor"
> op <- par(bg = "bisque")
> matplot(c(1, 8), c(0, 4.5), type = "n", xlab = "Length", ylab = "Width",
> main = "Petal and Sepal Dimensions in Iris Blossoms")
> matpoints(iris[iS,c(1,3)], iris[iS,c(2,4)], pch = "Ss", col = c(2,4))
> matpoints(iris[iV,c(1,3)], iris[iV,c(2,4)], pch = "Vv", col = c(2,4))
> legend(1, 4, c(" Setosa Sepals", " Setosa Petals",
> "Versicolor Sepals", "Versicolor Petals"),
> pch = "SsVv", col = rep(c(2,4), 2))
> Regards,
> Jeff Dick
Thanks a lot, Jeff.
This is an absolute record find in "R history":
This example has been *unchanged* in the matplot.Rd file already
in R's svn rev 2 -- file date 18. Sep 1997
-- almost 2.5 years before R version 1.0.0
i.e., more than 28 years ago.... and you did properly find an
error, indeed, even though only in the plot labeling.
I will fix it slightly differently than you, keeping the legend
identical, and using slightly more "principled" code....
basically this:
PS <- c("Petal", "Sepal") ; clr <- c(2,4)
psL <- paste(PS, "Length",sep="."); Sps <- paste0(" Setosa ", PS, "s")
psW <- paste(PS, "Width", sep="."); Vps <- paste0("Versicolor ", PS, "s")
matplot(c(1, 8), c(0, 4.5), type = "n", xlab = "Length", ylab = "Width",
main = "Petal and Sepal Dimensions in Iris Blossoms -- new version" )
matpoints(iris[iS, psL], iris[iS, psW], pch = "sS", col = clr)
matpoints(iris[iV, psL], iris[iV, psW], pch = "vV", col = clr)
legend(1, 4, c(Sps, Vps), pch = "sSvV", col = rep(clr, 2))
---
Thanks again, Jeff!
Martin
--
Martin Maechler
ETH Zurich and R Core team
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